The Japanese midfield star was on the bench for the disastrous defeat at Old Trafford, which marked the first time in 11 years that United have lost consecutive home games in the league.
The reason wasn't a hamstring strain or an ankle knock, however: instead, a bit of over-eating appears to have been to blame.
Not just any over-eating, either: indulgence heavy enough that the 24-year-old needed to go to hospital to have his stomach pumped.
Normally this is the kind of story that seems so fantastical that Early Doors would steer well clear - but the source is David Moyes speaking to Manchester United's official website.
"He thinks he ate too much, and he had to get his stomach pumped," Moyes said.
The manager added that Kagawa's illness had started earlier in the week:
"Shinji was very sick after the [Everton] game and he did a bit of training on Friday but he was low. All of the stats that we look at were low so we felt we had better not take a risk in case we got an injury by playing him.
"It was a bad sickness."
Of course, it wasn't just Kagawa feeling sick after that Everton match. Just about everyone with any stake or interest in Manchester United probably felt like throwing up after witnessing the Toffees' first win at Old Trafford since 1992.